On a fully working box at the grub> prompt 'ls (hd0,2)', lists the centos7
/boot files, and 'ls (hd0,3)' lists the centos6 /boot files.

On the recalcitrant box at the grub> prompt 'ls (hd0,2)', lists the centos7
/boot files, but 'ls (hd0,3)' says "error: unknown filesystem."

looking further, on the fully working box parted shows ext3 under 'File
system' (and raid under 'Flags') for sd[ab]3, but on the other parted shows
nothing under 'File system' (and raid under 'Flags') for sd[ab]3.

interesting that grub clearly has no trouble fetching grub.conf to present
the legacy_configfile menu, but when it comes to fetching the files in the
grub.conf entries, from the same filesystem, it won't.
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